[MF3D.FORUM:1489] flash distances...

["Bill Glickman" <bglick@xxxxxxxx>]

David

> That is not the case. The only thing that matters is the flash
> to subject distance. If you have a camera at zero feet, a
> flash at 10 feet, and a subject at 20 feet the GN and all
> settings would be no different that if the camera were at 10
> feet with the flash or for that matter at minus twenty feet.

      I would have to strongly challenge this David.  I would like to know
how you came up with this....I am very curious...

         This entire issue boils down to the total
"flash-to-subject-to-film" distance.  In your first scenario, the total
distance is 10 ft + 20 ft = 30 ft total travel distance.   In your next
scenario you have 40 ft + 10 ft = 50 ft.   This is a very big difference in
how much the light has to travel.  The inverse square law would clearly
apply here...   If you change one variable, you effect the outcome.   In
addition to the inverse square law which states the further the distance -
the more light scatters, so less returns back to the film....there is some
other very important issues to consider, because photographing in the field
rarely mimic lab testing of flat subjects.

       For example, although a flashes light can be controlled when leaving
the flash, once it hits the subject, its' scatter is random, based on the
reflectivity and angles of the subject in relationship to the position of
the camera....  So this factor also has a huge effect on the scattering of
light making it back to the film.  In addition, just the simple difference
of a white or a black subject color will effect the light coming back to the
film by 5 stops!  That is 32x more light coming back if the subject is white
vs. black..... this is why it is almost impossible to accurately calculate
these things, a good spot flash meter and a lot of Polaroid's is your best
bet.  If you get good enough with the spot meter, you can stop wasting
Polaroid's, but there is still always something the Polaroid's see that we
miss!

           The inverse square law will always apply considering all other
things remain constant....so the cheat sheet values I pasted earlier, which
are simplified inverse square laws given in stop values, give one a good
feel how much something needs to be changed to meet your objectives...this
way we can reduce set up time!

Bill G